Known methods of lighting a discharge lamp include a square-wave lighting method and a sinusoidal-wave lighting method, where it is necessary to detect the state of the discharge lamp, particularly, a lamp voltage to control properly power to be applied to the discharge lamp, and to determine correctly an error and the like-based on a detection signal indicative of a detected lamp voltage.
To prevent a re-arc voltage which occurs upon inversion of the polarity of a voltage across a discharge lamp from directly affecting the result of a detected lamp voltage, a period in which the re-arc voltage occurs may be excluded from a detection period.
For example, a method of masking a re-arc voltage may involve generating a timing signal in such a manner that no sampling is performed during a period in which the re-arc voltage can occur in a sample/hold circuit. In other words, a lamp voltage is sampled and held only during a period in which the re-arc voltage is absent, and is presented as the detection result.
For this purpose, it is necessary to identify a time at which the lamp voltage is not sampled, based on some signal, i.e., a signal for masking the re-arc voltage. In regard to AC lighting of a discharge lamp, semiconductor switching elements are employed in a DC-AC converter and the like, with the ability to control on/off operations of the switching elements, so that it is possible to detect a level change switching timing (H→L or L→H) associated with a driving signal for the element, or to detect the time from a signal which is the basis of the driving signal (output signal of an oscillator circuit or the like) to generate a mask signal (signal for masking) for the re-arc voltage.
Some conventional circuit configurations have a problem in that no specific and effective actions are taken for masking the re-arc voltage without fail upon detection of the lamp voltage in the high frequency AC lighting.
In a lighting circuit for a discharge lamp, a driving signal from a control circuit is supplied to the switching elements for controlling the switching of these elements, or depending on the circuit configuration, a voltage is supplied to a discharge lamp through a resonator circuit, a transformer and the like, where the voltage across the discharge lamp has its polarity inverted at a period corresponding to a switching frequency. Since the timing of inverting the polarity of the voltage across the discharge lamp is delayed from a change in the driving signal to the switching elements at all times, a mask signal for the re-arc voltage should be generated in view of this delay.
A shift in timing due to such a delay can be readily adjusted within a low lighting frequency range such as several hundreds kilohertz, but the situation becomes more serious as the lighting frequency increases. Specifically, as the switching element driving frequency increases, a larger shift in relative timing occurs between the time at which the driving signal changes and the time at which the polarity is inverted, thus causing difficulties in correctly fitting the mask signal generated based on the change in the driving signal to a period in which the re-arc voltage occurs. As a result, the lamp voltage cannot be correctly detected (a larger timing shift causes an adverse influence due to the detection of the re-arc voltage to exert on the detected lamp voltage), possibly hampering the power control of the discharge lamp, and the like.
It is, therefore, desirable to generate correctly a mask signal for a re-arc voltage in view of a support to high frequency lighting in a lighting apparatus for a discharge lamp such that the re-arc voltage will not affect the detection of a lamp voltage, thus accurately detecting the lamp voltage.